1. Field of the Invention
The invention relates to the field of optics and more particularly to waveguides and methods of forming waveguides.
2. Description of Related Art
A fiber optic waveguide is generally used in the transmission of speech, data, pictures, or other information by light. An optical communication system is a system that utilizes an information-carrying light wave signal that originates in a transmitter, passes through an optical channel, and enters a receiver that reconstructs the original information.
A fiber optic waveguide is conventionally a cylindrical structure of two indices of refraction, an inner core and an outer cladding. The inner core has a first index of refraction that is typically greater than the index of refraction of the outer cladding. Light travels through the fiber by a process of total internal reflection wherein the light is restricted or guided through the fiber in a single dimension. In other words, a fiber waveguide restricts light in two dimensions, the two directions that are orthogonal to the desired direction, so that light will internally reflect and travel in the remaining direction.
There are basically two types of fiber optical material. The first is a glass fiber used commonly to carry light a considerable distance. Another type of fiber is a polymer-based fiber that is, generally, considerably less expensive than glass fiber optical material. A polymer-based fiber optic is, generally, produced in one of two ways. The first method is by a coaxial extrusion process. In this process, a first polymer material of typically a high index of refraction is put through an extruder to produce a small inner fiber that is the core. A second material of lower index of refraction, is then displaced around the inner core to form the cladding. Typically this is accomplished by placing the inner core in a melt of the lower index of refraction material to place the cladding about the core.
Another method of making a polymer-based fiber optic is by a diffusion method. In this process, an initial fiber form, typically of a high index of refraction, is created by an extrusion process. Next, a second material of lower index of refraction is placed around the initial form and caused to diffuse into the form, thereby modifying the refractory properties of the outer portion of the inner form. The fiber created by this process is known as a graded-index of refraction material. The amount of diffusion varies with the time allotted for the diffusion process and the materials used. Thus, a graded index multi-mode fiber is a fiber where the core refractive index decreases with increased radial distance. This method is detailed in Y. Outsuka, Appl. Phy. Lett., 23(5), 247 (1973). In Outsuka, a cross-linked monomer, diallylisophthalate or diethylene glycol bis-allylcarbonate, is partially polymerized to a gel and immersed or exposed to a monomer with a lower index of refraction, such as methyl methacrylate. The monomer gets incorporated into the gel by diffusion and polymerizes to form the outer cladding.
Other ways of making polymer-based optical fibers include thermally activated solvent-assisted infusion of a monomer into polymethyl methacrylate, an ultraviolet-based polymerization.
Another type of optical material used to carry light waves, typically a shorter distance, is a planar optical sheet. The conventional use for such a sheet is in a planar optical display ("POD") device. A POD utilizes a number of optical sheets stacked one on top of each other, each sheet carrying different optical information to convey the information from a transmitter to a receiver. The optical sheet is a planar version of the optical fiber, in that the sheet guides light by the same optical process used in fiber optics. Unlike the optical fiber, however, which restricts the travel of light in two dimensions (i.e., the two dimensions orthogonal to the desired travel of the light), the planar optical sheet restricts light in only a single dimension. FIG. 1 illustrates a sheet of optical material 5 that is used, for example, in a planar optical display device. FIG. 1 illustrates a schematic cross-sectional side view of optical sheet 5. The sheet of optical material 5 is constructed similar to an optical fiber as discussed above and contains an inner core material 10 of a high index of refraction polymer material and an outer cladding material 20 of a polymer having a lower index of refraction. In FIG. 1, a wave of light 100 is restricted only in the Y-direction, thus leaving the X- and Z-directions available for travel. Thus, an optical signal sent along the X-direction can spread out in the Z-direction as it travels through the sheet of optical material 5.
FIG. 2 illustrates a schematic top perspective view of a portion of optical sheet 5 presented in FIG. 1. FIG. 2 shows that a wave of light 100 propagated in the X-direction can spread out in optical sheet 5 in the Z-direction. Wave of light 100 is restricted from traveling in the Y-direction by outer cladding 20.
Optical sheet 5 is produced in a manner similar to the production of optical fibers discussed above. The two main methods of producing optical sheet 5 are the step-index and graded-index. To produce a step-index of refraction optical sheet by a co-extrusion method a hot polymer material of high index refraction is directed through a die extruder where it spreads out into a plastic sheet or film. A second material having a lower index of refraction relative to the higher index of refraction core is co-extruded on the top side and on the bottom side of the polymer sheet or film. The step-index of refraction is created by a distinguishable demarcation between the index of refraction of the first material and the second material.
A graded-index of refraction material is created by forming an inner core sheet or film of a polymer material having a high index of refraction and overlying and underlying that material with a second polymer material or coating having a lower index of refraction relative to the index of refraction of the core sheet or film. In this case, a diffusion process takes place wherein a portion of the lower index of material polymer diffuses into the top and bottom sides of the core polymer sheet or film.
FIG. 3 schematically illustrates a display assembly utilizing a POD device. The use of a POD device for displaying an image has advantages over conventional lens/mirror arrangements. For example, in many applications, especially when a relatively large display must fit in a compact enclosure, POD devices offer better depth of field focus characteristics than lens/mirror arrangements. With lens/mirror arrangements, a reflected image is spread out about, for example, a display screen such that portions of a display screen image will be closer to the reflected image than other portions of the display screen image. This creates depth of field focus issues that are addressed by the use of additional mirrors. With a POD device, an image is focused on the bottom of the POD device and is displayed at the other end of the POD in the same focus and without depth of field focus issues.
In FIG. 3, POD 40 is made up of a plurality of optical sheets 30 laminated together and formed into a triangular or wedge-shaped device. The assembly in FIG. 3 is, for example, a laser scanning system where a laser 50 generates a display by scanning and modulation. The laser sweeps an image and paints the image to a viewer of a POD display. Laser 50 sends image (light waves) 100 to modulator 60. Image 100 is reflected off scanning mirror 70 and projected by scanning mirror 70 at the base of POD 30 into POD 30. From there, image 100 is carried up through the POD layers or optical sheets 40 and projected to a viewer of the display system. The use of the individual layers or optical sheets 30 keep the laser light images separate from one another so that a sharp image is displayed to the viewer. Laser generated image 100 is projected onto the bottom of POD 40 in sharp focus in a direction orthogonal to the layers or optical sheets 30 because laser 50 prevents the light from spreading. Once image 100 is transferred to POD 40, however, the image is restricted in only one orthogonal direction, thus allowing the light waves of the image to spread out. The use of POD 30 in a laser scanning system is acceptable because the laser produces single frequency, well-collinated light and the margin of dispersion or spreading out of the light in an undesirable orthogonal direction once in POD 40 is acceptable for many uses. However, such would not be the case in, for example, lens/mirrors systems or other systems that do not use well-collinated light.
What is needed is an optical sheet capable of restricting the travel of light in two directions.